An electro-active polymer (EAP) is a soft actuator that can generate high stress and large strain. A dielectric elastomer actuator (DEA) is an EAP which uses a dielectric elastomer film or membrane with a flexible electrode attached to each face. Application of a voltage difference across the electrodes generates Maxwell stress. The two electrodes are urged together by electrostatic attraction of free charges on the electrodes, causing the thickness of the membrane to decrease and the surface area of the faces to increase. This deformation is further caused by electrostatic repulsion of like charges on each individual face. Thus, application of a voltage to the electrodes causes the membrane to expand in area and compress in thickness.
A dielectric elastomer generator (DEG) is a class of electrostatic generator that is similar in configuration to a DEA except that it uses an EAP to convert mechanical energy into electrical energy. In principle at least, a DEG is essentially a DEA working in reverse.
Pelrine et al. in “Electrostriction of polymer dielectrics with compliant electrodes as a means of actuation”, Sensors and Actuators A 64, 77-85 (1998) describe the basic principles behind DEAs and a linear actuator based thereon.
US Patent Application No. 2002/0008445 describes arrangements in which an electroactive polymer may be used in a transducer to convert electrical energy into mechanical energy by applying a voltage to electrodes contacting the electroactive polymer. The transducer may also be used to convert mechanical energy into electrical energy by mechanically deflecting the electroactive polymer.
For many years, electrical transformers operating using the principles of electromagnetic induction have been used to ‘step up’ or ‘step down’ voltage from one circuit to another. Electromagnetic transformers are known to be inefficient at certain, especially low, frequencies. They typically require metallic components and create magnetic ‘noise’, neither of which are desirable in some applications. Transformers have a characteristic ‘humming’ noise which may be undesirable.
Piezoelectric transformers are transformers which operate using the principles of piezoelectricity. In one form, piezoelectric transformers operate on the basis of acoustic coupling between input and output. By applying an input voltage to a piezoelectric material it can be made to vibrate and, at the appropriate frequency, resonate. A higher output voltage can therefore be generated at another section of the material.
In another form, such as discussed in U.S. Pat. No. 3,487,239, piezoelectric transformers consist of a motor portion and a generator portion. An electrical field applied to the former is converted into mechanical energy using the piezoelectric effect. The mechanical energy is input into the generator portion to convert the mechanical energy back into electrical energy, again by the piezoelectric effect.
Piezoelectric transformers are known to have several characteristics which are not always desirable, depending on the application. These include: low efficiencies; poor performance at low frequencies; limited to low amplitude applications; resonance problems; voltage drift if the transformer is run at low frequencies; and the production of only alternating current (AC) power.